INFLUENCE OF THE MAXIMUM TEMPERATURE DURING PARTIAL MELT-PROCESSING OF BI-2212 THICK-FILMS ON MICROSTRUCTURE AND J(C)

The critical current density j(c) of Bi-2212 thick films (d = 60 mu m) depends on the maximum processing temperature T-max during the partia l melting process. Processing at T-max = 893 degrees C leads to the hi ghest j(c) of 3500 A/cm(2) (77 K, 0 T). A variation of T-max by +/-5 K from the optimum temperature leads to a drop of j(c) below 80% of the maximum value. Processing below the optimum T-max leads to insufficie nt consolidation. Partial melting at T-max = 904 degrees C leads to 5 vol% more residual peritectic phases with an average grain size about twice as large in the final microstructure compared to optimally proce ssed samples. However, the drop of j(c) in samples processed at higher T-max is attributed to inhomogeneities in the microstructure. These a reas have a reduced film thickness and consist of second phase grains, pores, and misaligned 2212 platelets. The more frequent formation of these inhomogeneities is attributed to inhomogeneous nucleation of the Bi-2212 platelets upon slow cooling at a rate of 5 K/h from the parti ally molten state. In optimally processed thick films the nucleation o f the Bi-2212 platelets occurs regularly in the whole cross section of the film. If processed at higher T-max, fewer nucleation sites are pr esent, leading to regions of high platelet density and regions contain ing no Bi-2212 grains. Due to the high capillary forces in the narrow gap between the Bi-2212 platelets, liquid is transported away from the platelet-free regions and stored between the Bi-2212 grains. Because of this phase separation, the solid peritectic grains persist in the a reas where liquid was extracted. (C) 1998 Elsevier Science B.V.